Prior-art shock absorber and level controller fluids, particularly used in vehicles, are select mineral oil fractions in combination with suitable lubricant additives. Said fluids are also termed in the following `automotive hydraulic fluids`. Generally, these fluids meet the requirements with respect to shock absorption and ageing resistance. However, their biodegradation is not sufficiently rapid and their ecotoxicological characteristics are unsatisfactory so that they have a considerably polluting impact on the environment. Since, particularly with shock absorbers, uncontrollable oil leakage frequently occurs, environmentally acceptable shock absorber fluids are badly required.
Therefore, it was first attempted to substitute the petroleum products in question with natural materials, e.g. vegetable oils.
PCT publication WO 88/08808 describes hydraulic fluids based on natural triglycerides and a combination of vegetable oils with synthetic esters as adjusting agents for lowering or increasing the viscosity. However, the low-temperature behavior of these products, too, remains significantly above the values required of automotive hydraulic fluids.
For instance, rape and sunflower oil have high lubricating powers and excellent viscosity temperature (VT) behaviors, whereas their ageing and low-temperature properties are unsatisfactory so that these materials were inappropriate as substitutes.
Polyethylene glycols are used to a limited extend in universal industrial hydraulic applications, but such fluids are not suitable for automotive applications because their low-temperature fluidities and compatibilities with conventional lubricants are unsatisfactory.
Finally, for use in shock absorbers, saturated dicarboxylic acid esters and polyol esters have been examined which, although they presented yet sufficient degradation rates and good thermal stabilities, had viscosity indices (VI) which were significantly lower than the values obtained with natural oleic acid esters. Furthermore, when using NBR (nitrile butadiene rubber), a material which is normally used in technical applications, the elastomer swelling behavior is inacceptable.
U.S. Pat. No. 2,757,139 of the year 1953 discloses synthetic ester oils as lubricants for use in aircraft gas turbines. The experimental part of said patent publication only relates to esters of coconut fatty acids with C.sub.16, C.sub.26 Guerbet alcohols tested as turbine lubricating oils. The esters described in said publication have insufficient VI indices for the application of the invention (max. 148) so that considerable amounts of VI improvers have to be added to reach values of higher than 150. In addition, the fluidities and low-temperature behaviors of said esters are unsatisfactory. No ecological requirements are made on the products.